The invention concerns a switchable flat or roller tappet for transmitting a cam lift to a push rod in a valve train of an internal combustion engine, said tappet comprising a housing which can be mounted with an outer peripheral surface in a bore of the internal combustion engine, said housing comprising an axially extending cavity in which an inner element that is longitudinally displaceable relative to the housing is inserted, each of the housing and the inner element comprising at least one reception for at least one coupling means, said receptions being aligned to each other in a relative position for which the coupling means can be displaced so as to extend partially in each of said receptions for coupling the housing to the inner element, a first body being arranged between the housing and the inner element to positively prevent a rotation of the inner element relative to the housing, and a second body being arranged on the outer peripheral surface of the housing to positively prevent a rotation of the housing relative to the bore of the internal combustion engine.
U.S. Pat. No. 5,361,733 discloses a tappet of the pre-cited type that is considered to be generic.
FIGS. 2, 4a of this patent show a tappet whose one end is associated to a group of cams and acts in the region of its other end directly on an end of a push rod. In the embodiment of FIG. 2, a rotation of the housing relative to the displaceable inner element is prevented by a pin that is fixed in the housing and extends radially inward into a longitudinal groove of an inner element that is surrounded by the housing. On the one hand, the pin prevents a rotation of the housing relative to the inner element and serves on the other hand as an upper axial stop for the inner element relative to the housing.
A separate rotation-prevention device for the housing relative to its bore in the internal combustion engine or in a component associated to the internal combustion engine is disclosed in the embodiment of FIG. 4a. The axle pins of this roller tappet extend radially outward beyond the housing. The protruding parts of the axle pins serve to prevent a rotation of the housing relative to the surrounding bore.
A drawback of the aforesaid tappet is that its inner and outer rotation-preventing measures are relatively complex. The need for two separate components unnecessarily increases costs and assembly work.
It is an object of the invention to create a tappet of the pre-cited type that is free of the mentioned drawbacks.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects in a tappet of the initially cited type by the fact that the first and the second body are made as a single component that is fixed in one of the inner element and the housing, a portion of the component extends into an opposing slot of the other of the inner element and the housing, said slot being positioned so as to form together with said portion of the component an upper stop for defining the relative position.
Thus, one single component, which in a first embodiment of the invention is made as a piston, serves both as an inner and an outer rotation-preventing device in the tappet. This unification of the otherwise two separate rotation-preventing devices in a single component simplifies the assembly of the tappet and at the same time reduces the overall costs.
In a preferred embodiment of the invention, the component extends in the reception of the inner element that at the same time serves as a reception for the coupling means. The scope of the invention extends also to solutions in which the component is arranged at a different height level from the reception of the inner element or of the housing. It is also possible to arrange the component fixedly in the housing in which case the slot is provided in the inner element.
If the reception in the inner element is made as a throughbore, an inner end of the component can serve as a support for one end of a compression spring whose opposing end acts on the coupling means for effecting coupling. Thus separate supporting measures for the compression spring can be dispensed with. The tappet is configured so that coupling is achieved by the spring force of the compression spring and uncoupling, through hydraulic medium conducted to an outer end of the coupling means. It is also conceivable to reverse this principle or to apply hydraulic medium pressure to both ends of the coupling means.
To limit the travel of the coupling means in radially outward direction for effecting coupling, it is proposed to make the reception for the coupling means in the housing with a stepped configuration so that the coupling means is stopped on the thus implemented diameter reduction.
According to an advantageous feature of the invention, a hydraulic clearance compensation element is installed within the tappet so that known mechanical adjusting measures are not required in the valve train.
According to a further proposition of the invention, the tappet is configured as a disconnectable tappet. For this, the housing comprises a contacting surface for only one cam. However, the invention can be equally well used in tappets cooperating with variable lift cams as discussed in connection with the prior art cited above.
The invention also proposes a particularly low-friction cam contact realized by the fact that the cam-proximate end of the housing comprises a roller. This roller can be mounted through a rolling or a sliding bearing.
According to a final proposition of the invention, an outer portion (end) of the one-piece component which, to prevent a rotation of the entire tappet, can be arranged in a longitudinal groove of the bore that surrounds the tappet, has a crowned configuration. However, it is also within the scope of the invention to make this outer portion cylindrical in shape with an axial line of this cylinder extending parallel to the longitudinal axis of the tappet. This has the advantage that the contact of the outer portion with the longitudinal groove is a linear contact and this reduces Hertzian stress. The cylinder may be made as a separate element and be connected to the component, for example, by gluing, but it can also be an integral part of the component.